(1) Field of the Invention
The present invention relates to developing devices and image forming apparatuses, and in particular to a technology of preventing an image formation failure occurring in a developing device using a two-component developer due to the change in the ratio between the amount of toner and the amount of magnetic carrier.
(2) Related Art
A developing device used in an image forming apparatus develops an electrostatic latent image formed on the outer circumferential surface of the photosensitive drum by using toner stored in the device. The amount of the consumption of the toner depends on the area and the density of the electrostatic latent image. In particular, when a developing device using two-component developer containing toner and magnetic carrier develops an electrostatic latent image that requires a large amount of toner, the amount of the toner stored in the developing device greatly decreases, and accordingly the ratio of the toner in the developer (hereinafter referred to as “T/C ratio”) greatly decreases. If the fluctuation range of the T/C ratio is large, the amount of charge per unit weight of the developer (hereinafter referred to as “toner charge amount”) and the magnetic property and flowability of the developer also greatly change, and accordingly the amount of the developer transported by the developing sleeve greatly changes as well.
If the amount of the developer transported by the developing sleeve becomes too much according to the change in the T/C ratio, it would be possible that a portion of the developer gets stuck between the developing sleeve and the photosensitive drum. If developing is forcibly performed with the portion of the developer being stuck, various problems could occur. For example, the carrier might adhere to the photosensitive drum or a recording sheet, or the developer might be scattered in the device and adhere to other components of the device.
However, if the minimum distance DS between the developing sleeve and the photosensitive drum is increased to prevent the developer from getting stuck, it will be necessary to increase the electric field intensity between the developing sleeve and the photosensitive drum by increasing the bias voltage for development applied to the developing sleeve, in order to cause electrostatic attraction of charged toner to the photosensitive drum. This increases the edge effect of the electric field, and causes an image formation failure. FIG. 27 is a conceptual diagram illustrating the edge effect. As shown in FIG. 27, if the electric field intensity is increased when the minimum distance DS is large, the toner will be supplied to the edge of the electrostatic latent image from a larger area of the developing sleeve. Consequently, an image formation failure occurs, that is, the concentration of the toner in the edge becomes too high (FIG. 27B).
As a solution to this problem, there has been known a technology of adjusting the amount of developer transported by the developing sleeve by controlling the positional relationship between a restriction member, which restricts the amount of developer transported by the developing sleeve, and a magnet roll, which is inserted in the developing sleeve, according to the T/C ratio of the developer detected by a magnetic permeability sensor, for example (See Japanese Patent Application Publication No. 10-31366, for example). There has also been known a technology of controlling the amount of transportation of developer by controlling the distance between the restriction member and the developing sleeve according to the T/C ratio detected in a similar manner (See Japanese Patent Application Publication No. 09-222792, for example). With such optimization of the transportation amount of developer, it is possible to prevent the developer from getting stuck without increasing the minimum distance DS between the developing sleeve and the photosensitive drum, and thereby prevent problems such as image formation failures.
However, the use of a magnetic permeability sensor for detecting the T/C ratio is undesirable, because it increases the component cost and manufacturing cost of the developing device, contrary to demands for cost reduction of developing devices and image forming apparatuses. In addition, it will be necessary to provide a mechanism for controlling the distance between the developing sleeve, or the magnet roll inserted in the developing sleeve, and the restriction member. In terms of such a necessity, the use of a magnetic permeability sensor inevitably increases the cost.